Process for the expoxidation of olefins

ABSTRACT

The invention described herein relates to a process for the catalytic epoxidation of olefins with hydrogen peroxide in a continuous flow reaction system, wherein a gaseous phase containing an olefin and a liquid phase containing the hydrogen peroxide is present in the reaction system and the gaseous phase flows in countercurrent to the liquid phase.

This application is a 371 of PCT/EP01/01167, filed Feb. 3, 2001.

PRIOR ART

From EP-A 100 119 it is known that propene can be converted by hydrogenperoxide into propene oxide if a titanium-containing zeolite is used ascatalyst.

Unreacted hydrogen peroxide cannot be recovered economically from theepoxidation reaction mixture. Furthermore, unreacted hydrogen peroxideinvolves additional effort and expenditure in the working up of thereaction mixture. The epoxidation of propene is therefore preferablycarried out with an excess of propene and up to a high hydrogen peroxideconversion. In order to achieve a high hydrogen peroxide conversion itis advantageous to use a continuous flow reaction system. Such areaction system may comprise either one or more tubular flow reactors oran arrangement of two or more flow mixing reactors connected in series.Examples of flow mixing reactors are stirred tank reactors, recyclereactors, fluidised bed reactors and fixed bed reactors with recyclingof the liquid phase.

In order to achieve a high reaction velocity as high a propeneconcentration as possible in the liquid phase is necessary. The reactionis therefore preferably carried out under a propene atmosphere atelevated pressure.

The decomposition of hydrogen peroxide with the formation of molecularoxygen always occurs to a slight extent as a secondary reaction on thetitanium silicalite catalyst. In order to be able to operate theepoxidation process reliably on an industrial scale the oxygen that isformed must be removed from the reaction system. This is effected mostsimply by flushing the oxygen out with a propene waste gas stream.

EP-A 659 473 describes an epoxidation process that combines thesefeatures. In this connection a liquid mixture of hydrogen peroxide,solvent and propene is led over a succession of fixed bed reaction zonesconnected in series, wherein the liquid phase is removed from eachreaction zone, is led over an external heat exchanger to extract theheat of reaction, and the major proportion of this liquid phase is thenrecycled to this reaction zone and a minor proportion of the liquidphase is passed to the next zone. The individual reaction zones behaveas flow mixing reactors on account of the liquid recycling over thefixed bed. At the same time gaseous propene is fed in together with theliquid feed stock mixture, is guided in a parallel stream to the liquidphase over the fixed bed reaction zones, and is extracted at the end ofthe reaction system in addition to the liquid reaction mixture as anoxygen-containing waste gas stream. Although this reaction procedureenables the propene oxide yield to be raised compared to conventionaltubular reactors without the temperature control described in EP-A 659473, it nevertheless involves considerable additional costs on accountof the complexity of the reaction system required to carry out theprocess. Furthermore, the described raised yield can only be realised ifthe propene oxide contained in the waste gas stream is recovered. Thisnecessitates an additional process stage, which in turn adds further tothe costs of the process.

The object of the present invention is accordingly to provide a simpleinexpensive process for the epoxidation of olefins with hydrogenperoxide, with which high conversions can be achieved combined with ahigh product yield and which can be carried out using conventionalreaction systems.

SUBJECT OF THE INVENTION

This object is achieved by a process for the catalytic epoxidation ofolefins with hydrogen peroxide in a continuous flow reaction systemwherein a gaseous phase containing an olefin and a liquid phasecontaining the hydrogen peroxide are present in the reaction system andthe gaseous phase is fed in countercurrent to the liquid phase.

An important advantage of the countercurrent arrangement according tothe invention is the reduction in the amount of propene oxide that isdischarged from the reaction system together with the oxygen-containingpropene waste gas stream, and the resultant decreased expenditure onrecovering propene oxide from this waste gas stream. As small a loss ofpropene oxide as possible is desired in order to achieve a high productyield according to the invention.

The countercurrent arrangement according to the invention of gaseousolefin and liquid reaction mixture in the reaction system may beaccomplished in various ways depending on the chosen reaction system. Inthis connection reaction systems are suitable in which there is nocomplete back-mixing relative to the overall system, i.e. reactionsystems whose residence time spectrum exhibits a maximum, or reactionsystems involving plug flow.

If the epoxidation of olefins is carried out in a tubular flow reactor,then the gas stream containing the olefin is guided in countercurrent tothe liquid phase within the reactor. In this connection the liquidstream is preferably led from the top downwards through the reactor,while the olefin flows from the bottom upwards through the reactor inthe form of a gas stream. The reactor may be operated as a bubble columnwith a continuous liquid phase, as well as a trickle reactor with acontinuous gas phase. The catalyst may be employed either as asuspension in the liquid phase or in the form of a fixed bed, whereinthe fixed bed may be designed both as a random catalyst packing as wellas an ordered packing of coating monoliths or distribution bodies.Preferably a tubular flow reactor is used as a fixed bed reactor with arandom catalyst packing and continuous liquid phase.

In order to be able to operate the process continuously when changingand/or regenerating the epoxidation catalyst, two or more tubular flowreactors may if desired also be operated in parallel or in series in theaforedescribed manner.

If the epoxidation of olefins is carried out in a succession of two ormore tubular flow reactors connected in series, the substance streams ofliquid phase and gaseous phase within a flow reactor may be guidedeither in co-current or in countercurrent, the substance streams beingguided in countercurrent between the tubular flow reactors.

In an alternative embodiment the reaction system may comprise severalreactors connected in series that are chosen independently of oneanother from flow mixing reactors and tubular flow reactors, thesubstance streams of liquid phase and gaseous phase being guided incountercurrent between the reactors. For example, flow mixing reactorsand tubular flow reactors may also be used in combination within thereaction system consisting of reactors connected in series. Preferably,in this connection one or more flow mixing reactors are connected inseries with a final tubular flow reactor. The particular advantage ofsuch a reaction system is that the heat of reaction can be particularlyeasily extracted from the flow mixing reactors in which the majorproportion of the reaction turnover takes place. The use of a finaltubular flow reactor ensures that the hydrogen peroxide conversion takesplace as fully as possible. Stirred tank reactors, recycle reactors, jetreactors with liquid circulation, or fixed bed reactors with a liquidcirculation over the fixed bed are for example suitable as flow mixingreactors.

Using the process according to the invention olefins can be epoxidisedthat are at least partially in the gaseous phase under the chosenreaction conditions. This applies in particular to olefins with 2 to 6carbon atoms. The process according to the invention is mostparticularly suitable for the epoxidation of propene to propene oxide.

For economic reasons it would be preferred for an industrial scaleprocess to use propene not in a pure form but as a technical mixturewith propane that as a rule contains 1 to 15 vol. % of propane. Sincepropene is consumed in the epoxidation reaction, propane accumulates inthe gas stream during its passage through the reaction system, which inthe case of a co-current flow arrangement leads to a decrease in thereaction velocity and to differences in the generation of heat throughthe exothermal epoxidation reaction along the chain of reactors. Thesedisadvantages can be avoided by the countercurrent flow of gas phase andliquid phase according to the invention. Furthermore the propene oxideyield is raised even in the presence of propane in the feed streamcompared to a co-current feed with the countercurrent feed of thesubstance streams. From this it is clear that, by means of the processaccording to the invention, not only can a high turnover and a highpropene oxide yield be achieved with low expenditure on apparatus, butalso the use of technical propene with up to 15% of propane does nothave a deleterious effect on the reaction and the product yield. Theeconomy of the process according to the invention is further improved onaccount of the usability of relatively cheap starting materials.

Crystalline, titanium-containing zeolites of the composition(TiO₂)_(x)(SiO₂)_(1-x) where x is from 0.001 to 0.05 and having a MFI orMEL crystalline structure, known as titanium silicalite-1 and titaniumsilicalite-2, are suitable as catalysts for the epoxidation processaccording to the invention. Such catalysts may be produced for exampleaccording to the process described in U.S. Pat. No. 4,410,501. Thetitanium silicalite catalyst may be employed as a powder or as a shapedcatalyst in the form of granules, extrudates or shaped bodies. For theforming process the catalyst may contain 1 to 99% of a binder or carriermaterial, all binders and carrier materials being suitable that do notreact with hydrogen peroxide or with the epoxide under the reactionconditions employed for the epoxidation. Granules corresponding to EP-A893 158 are preferably used as suspension catalysts. Extrudates with adiameter of 1 to 5 mm are preferably used as fixed bed catalysts.

The hydrogen peroxide is used in the process according to the inventionin the form of an aqueous solution with a hydrogen peroxide content of 1to 90 wt. %, preferably 10 to 70 wt. % and particularly preferably 30 to50 wt. %. The hydrogen peroxide may be used in the form of thecommercially available, stabilised solutions. Also suitable areunstabilised, aqueous hydrogen peroxide solutions such as are obtainedin the anthraquinone process for producing hydrogen peroxide.

The reaction is preferably carried out in the presence of a solvent inorder to increase the solubility of the olefin, preferably propene, inthe liquid phase. Suitable as solvent are all solvents that are notoxidised or are oxidised only to a slight extent by hydrogen peroxideunder the chosen reaction conditions, and that dissolve in an amount ofmore than 10 wt. % in water. Preferred are solvents that are completelymiscible with water. Suitable solvents include alcohols such asmethanol, ethanol or tert.-butanol; glycols such as for example ethyleneglycol, 1.2-propanediol or 1,3-propanediol; cyclic ethers such as forexample tetrahydrofuran, dioxane or propylene oxide; glycol ethers suchas for example ethylene glycol monomethyl ether, ethylene glycolmonoethyl ether, ethylene glycol monobutyl ether or propylene glycolmonomethyl ether, and ketones such as for example acetone or 2-butanone.Methanol is particularly preferably used as solvent.

The process according to the invention for the epoxidation of olefins,preferably propene, is carried out at a temperature of −10° to 100° C.,preferably at 20° to 70° C. The olefin is preferably employed in excessrelative to the hydrogen peroxide in order to achieve a significantconsumption of hydrogen peroxide, the molar ratio of olefin, preferablypropene, to hydrogen peroxide preferably being chosen in the range from1.1 to 10. When adding a solvent the amount of solvent is preferablychosen so that only a liquid phase is present in the reaction mixture.The solvent is preferably added in a weight ratio of 0.5 to 20 relativeto the amount of hydrogen peroxide solution used. The amount of catalystemployed may be varied within wide limits and is preferably chosen sothat a hydrogen peroxide consumption of more than 90%, preferably morethan 95%, is achieved within 1 minute to 5 hours under the employedreaction conditions.

In a preferred embodiment of the process according to the inventionpropene is used that may contain between 0% and 15% of propane. Propenemay be fed as a liquid as well as in gaseous form into the reactionsystem. The amount of propene that is fed in is chosen so that under thereaction conditions in the reactors a gas phase is formed consistingpredominantly of propene, and so that from the first reactor of thereaction system a waste gas can be removed whose oxygen content liesoutside the explosion limits for propylene-oxygen mixtures. The pressurein the reaction system is preferably chosen to be between 50% and 100%of the saturated vapour pressure of propylene at the reactiontemperature.

The present invention will be illustrated hereinafter with the aid offigures and examples for the epoxidation of propene.

FIG. 1 shows a tubular flow reactor. Stream 1 denotes the feed stream ofthe liquid reaction phase, stream 2 denotes the outlet stream of theliquid reaction phase, stream 3 denotes the feed stream of the gaseouscomponent, and stream 4 denotes the waste gas stream. These designationsare also retained for the other figures.

FIG. 2 illustrates the flow of the substance streams for three reactorsconnected in series. The liquid feedstocks are fed with stream 1 intothe first reactor. From the first reactor the liquid reaction mixture isled via the streams 5 and 6 into the second and third reactors and isremoved in liquid form as stream 2 from the third reactor. Propene,optionally mixed with propane, is fed with the stream 3 into the thirdreactor and together with the streams 7 and 8 is led in gaseous form viathe second reactor into the first reactor. From the first reactor awaste gas stream is removed via the stream 4, the waste gas streamcontaining, in addition to unreacted propene and possibly propane, themolecular oxygen formed by decomposition of hydrogen peroxide during theepoxidation reaction.

FIG. 3 shows by way of example a system consisting of three stirred tankreactors connected in series for the epoxidation using a suspensioncatalyst, wherein the system is operated according to the invention withcountercurrent flow of liquid phase and propene gas, and the numberingof the substance streams agrees with FIG. 2.

When tubular flow reactors are connected in series, the flow of thesubstance streams within a reactor may take place both in countercurrentand in co-current.

FIG. 4 shows by way of example a system consisting of three fixed bedreactors connected in series with countercurrent flow within thereactors, wherein the substance streams of liquid phase and propene gasflow in countercurrent between the reactors in the manner according tothe invention.

FIG. 5 shows by way of example a system consisting of three fixed bedreactors connected in series with co-current flow within the reactors,wherein the substance streams of liquid phase and propene gas flow incountercurrent between the reactors in the manner according to theinvention. In both diagrams the numbering of the individual substancestreams agrees with FIG. 2.

FIG. 6 shows by way of example the combination of two stirred tankreactors with a bubble column reactor operating in co-current flow forthe epoxidation using a suspension catalyst, the substance streams ofliquid phase and propylene gas flowing in countercurrent between thereactors in the manner according to the invention. The numbering of theindividual substance streams agrees with FIG. 2.

EXAMPLE

In an arrangement consisting of two stirred tank reactors and a tubularflow reactor with an overall volume of 6.25 liters, which are connectedto one another corresponding to FIG. 6, 43 wt. % aqueous hydrogenperoxide solution is fed into the first reactor at a rate of 1045 g/h inparallel with a 2.0 wt. % suspension of titanium silicalite in methanolat a rate of 2630 g/h (stream 1). 1120 g/h of propene in gaseous formare fed at the same time from below into the third reactor (stream 3).The three reactors are thermostatically controlled at a temperature of65° C. and the pressure in all three reactors is maintained at an excesspressure of 15.0 bar by means of a pressure retention valve on the firstreactor. 215 g/h of unreacted propene with an oxygen content of 0.6 vol.% are removed at the pressure retention valve (stream 4). The hydrogenperoxide concentration is determined by redox titration and the contentsof propene oxide, 1-methoxy-2-propanol, 2-methoxy-1-propanol and12-propanediol are determined by gas chromatography at regular intervalsin the liquid reaction mixture (stream 2) removed from the thirdreactor. When the stationary operational state has been reached thehydrogen peroxide conversion is 96.8%, the propene oxide yield referredto converted hydrogen peroxide is 90.3%, and the propene oxideselectivity, calculated as the ratio of the concentration of propeneoxide to the sum of the concentrations of the products propene oxide,1-methoxypropanol, 2-methoxypropanol and 1,2-propanediol, is 94.5%.

What is claimed is:
 1. Process for the catalytic epoxidation of olefinswith hydrogen peroxide in a continuous flow reaction system, wherein agaseous phase containing an olefin and a liquid phase containing thehydrogen peroxide is present in the reaction system, characterised inthat the gaseous phase is guided in countercurrent to the liquid phase.2. Process according to claim 1, characterised in that the reactionsystem is selected from one or more tubular flow reactors connected inseries or in parallel.
 3. Process according to claim 2, characterised inthat the reaction system comprises several tubular flow reactorsconnected in series, the substance streams of liquid phase and gaseousphase flowing either in co-current or in countercurrent within a tubularflow reactor, and the substance streams flowing in countercurrentbetween the tubular flow reactors.
 4. Process according to claim 1,characterised in that the reaction system comprises several reactorsconnected in series that are selected independently of one another fromflow mixing reactors and tubular flow reactors, the substance streams ofliquid phase and gaseous phase flowing in countercurrent between thereactors.
 5. Process according to claim 1, characterised in that thecatalyst is suspended in the liquid reaction phase.
 6. Process accordingto claim 2, characterised in that the catalyst is used in the form of afixed bed.
 7. Process according to claim 1, characterised in that atitanium-containing zeolite is used as catalyst.
 8. Process according toclaim 1, characterised in that the olefin is propene.
 9. Processaccording to claim 8, characterised in that a propene feed stream isused that in addition contains up to 15 vol. % of propane.